Electrochemistry has been widely used for the measurement of various analytes in blood samples. In an electrochemical system, ion selective or sensing electrodes particularly sensitive to, for example, sodium, potassium, chloride, lithium or calcium ions are exposed to a patient sample. The electrical activity detected by the ion selective electrodes is proportional to the concentration of the corresponding analytes, yielding rapid, accurate and repeatable determinations of analyte concentrations.
A system marketed by the Eastman Kodak Company under the trademark Ektachem Model DT-60 utilizes a single use carrier or cassette employing ion selective electrodes for the measurement of the electrolytes in serum or plasma. The Kodak system requires the simultaneous pipetting of both sample and calibrator onto two separate sections of the cassette. The system measures the potential difference between the calibrator and sample to yield a measurement of an electrolyte in the sample. Although the Kodak system performs adequately, utilizing a single-use disposable cassette, the cassettes must be used on an analyzer that is itself both complex and expensive. Furthermore, because the system measures a potential difference between two sections of the cassette, each of the sections on any given cassette must perform exactly the same. This requirement presents a manufacturing challenge and can increase the cost of manufacturing and quality control.
Another approach to the measurement of electrolytes using ion selective electrodes is a system developed and marketed by SenTech Medical Corporation, now owned by Johnson and Johnson. The SenTech system employs a single-use plastic sensor card which carries several ion selective electrodes suitable for measuring, for example, calcium, sodium, and chloride as well as sample pH. A reservoir carried by the card includes a chamber containing a calibrator fluid and an empty chamber which receives a patient sample. In use, the patient sample is added to the reservoir and the card is inserted into an analyzer. The reservoir is rotated to a calibration position, allowing calibrator fluid to flow across the ion selective electrodes and providing a calibration value for the analyzer for each of the electrodes. Once the calibration values have stabilized and are recorded by the analyzer, the reservoir is rotated further to allow the sample to flow across the electrodes. The flow of sample is used to clear the calibrator fluid from the electrode surfaces and to present sample to the electrodes for measurement.
Unfortunately, the SenTech system requires the fabrication of a number of parts which then must be assembled into the completed cards, making each of the sensor cards expensive and increasing the cost per test. The calibrator/sample reservoir must be manually rotated in response to prompts from the instrument, thus requiring operator attention during the analysis. Also, because the flow of sample is used to clear the calibrator fluid from the electrode surfaces, calibrator contamination is possible if sample flow is uneven or includes entrained gas bubbles.
Thus, there is a need for a simple and inexpensive electrochemical system for the measurement of electrolytes or other analytes measurable with ion-selective electrodes. There is also a need for a system using an inexpensive analyzer and correspondingly inexpensive electrodes. Preferably, such a system should be completely automatic once the analyzer is loaded with the appropriate electrode and with a patient sample so that an operator need merely start the analysis cycle and may then attend to other matters while the analysis is performed.